Magnetic focussing device for an electron tube



y 1966 RYUZO ORUI 3,252,033

MAGNETIC FOCUSSING DEVICE FOR AN ELECTRON TUBE Filed March 29, 1963 R. ORU/ Attorney United States Patent 3,252,033 MAGNETIC FOCUSSING DEVICE FOR AN ELECTRON TUBE Ryuzo Orui, Tokyo, Japan, assignor to Company, Limited, Tokyo, Japan Filed Mar. 29, 1963, Ser. No. 269,095 Claims priority, application Japan, Apr. 25, 1962,

37/1634!) 3 Claims. (Cl. 313-84) This invention relates to a magnetic focussing device for a long electron beam such as may be found in a travelling-wave tube, and more particularly to a magnetic focussing device of the so-called immersed-field type which is composed of a combination of a uniform and a periodic magnetic focussing device.

As regards an immersed-field type magnetic focussing device used in a low-noise travelling-wave tube, description has been made in my copending patent application Ser. No. 164,752, filed January 8, 1962. A general magnetic focussing device which is not of the immersed-field type and which is provided with an input or an output waveguide has already been disclosed in US. Patent 2,991,382.

As has been described in, this patent specification, it is relatively easy in the case of a magnetic focussing device of the non-immersed-field type to produce a uniformly periodical magnetic field by providing the device with means for compensating that function of the cylindrical magnets which is interrupted by the insertion of the w'vegu ide. In the case of an immersed-field type device such as disclosed in the copending application, such compensation becomes complicated as compared with that of the patent. Although compensation for the disturbance introduced in the neighborhood of the junction of a uniform and a periodic magnetic focussing device has already been taken into consideration in the copending application, no complete compensation was given for such non-uniformity that occurs in the periodic magnetic field in case an input or an output waveguide is disposed through the periodic magnetic focussing device. Disposition of an input or an Nippon Electric Japan, a corporation of output waveguide through the periodic magnetic focussing device, however, is necessary both in the immersed-field type and the non-immersed-field type devices in using the travelling-wave tube in an amplifier or an oscillator.

Accordingly, the object of the invention is to provide a magnetic focussing device of the immersed-field type for a travelling-wave tube, wherein a periodic magnetic focussing device having an input or an output' waveguide put therethrough is arranged so as to produce a uniformly periodical magnetic field along the axis of the travellingwave tube and, in particular, the axis portion intersecting the waveguide.

The abovementioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a partial schematical axial sectional view of a travelling-wave tube having a magnetic focussing device of the invention; v

FIG. 1a shows distribution of the magnetic flux density along the axis of the travelling-wave tube;

FIG. 2 is an axial sectional view of a uniform magnetic foscussing device in a general immersed-field type mag- I netic focussing device;

FIG. 2a shows distribution of the magnetic flux density produced by'the uniform magnetic focussing device alone; FIG. 3 is a cross-sectional view taken on line 3-3 of FIG. 1;

FIG. 4 is a perspective view of a in the embodiment shown in FIG. l.

Referring to FIGS. 1 and 3, a magnetic focussing device 10 of the invention shown therein comprises a uniform magnetic focussing device 12 positioned coaxially of a bulb 11 of a travelling-wave tube and covering a portion 111 of the bulb 11 surrounding an electron gun 112, and a periodic magnetic focussing device 13 located coaxially of the bulb 11 and covering another bulb portion 113 circumscribing a slow-wave structure 114, such as a helical electrode. The uniform magnetic focussing device 12 in turn comprises an elongate annular permanent magnet or a pile of thin annular permanent magnets 121 and a pair of pole pieces 122 and 123 disposed in direct contact with :both ends of the magnet means 121. An input waveguide 14 is arranged 'contiguous'ly to the pole piece 123 which is on the side of the forward end of the bulb 11. Attached to the input waveguide 14 in turn is a pile of hollow cylindrical permanent magnets 13-1, 13-2, 133, and 134 of the periodic magnetic focussing device 13 which are oriented so that the senses of the every other magnetomotive force may be alternating, with thin annular pole pieces 135, 136, 137, and 13 8 interposed between the respective magnets 131, 132, 133', and 134.

As shown by a curve A in FIG. 1a in which the abscissas show points on the axis of the bulb 11 and the ordinates show the rightward magnetic flux density on the axis, the distribution of they magnetic flux density is, in ideal operation of the immersed-field type magnetic focussing device 10, flat at the portion of the uniform magnetic focussing device, while substantially sinusoidal at the portion of the periodic magnetic focussing device.

In the manner schematically illustrated in FIG. 2 and by a curve B in FIG. 2a, the magnetic field produced by the uniform magnetic focussing device alone includes magnetic field portions of the opposite sense, which are introduced by the counter magnetic fields of the magnet means 121, on the axis at outside portions of the pole pieces 122 and 123' of the magnet means 121. As has been pointed out in the copending application, it is'required to eliminate as far as possible any disturbance caused by one of the counter magnetic fields to the periodic magnetic field produced by the periodic magnetic focussing device. In the magnetic focussing device of the invenmagnetic shunt used tion, the pole piece 123 is of a relatively larger radius than the magnet to reduce the effect of the counter mag netic field. At the same time the cylindrical permanent magnet 131 placed nearest to the uniform magnetic focussing device is oriented so as to allow positive utilization of the counter magnetic field. For instance, the magnet 131 is so oriented that it may produce rightward magnetic field in the space within the bulb surrounded by such magnet and consequentlyleftward magnetic field running from the pole piece 135 to the pole piece 123, in the space within the bulb portion extending through the waveguide 14. The leftward magnetic field, even though weak, is strengthened by the counter magnetic field which is in the aiding sense. In order to reduce the magnetic reluctance of the magnetic circuit including the leftward magnetic field, the pole piece 136 to the right of the cylindrical magnet 131 is extended radially outwardly and connected by a magnetic shunt 15 with the pole piece 123. As shown in FIG. 4, the magnetic shunt 15 is made of a magnetic material in a shallow cup-like shape provided with a pair of notches15'1 for receiving the waveguide 14. The radius of the magnetic shunt 15 is determined in accordance with the magnitude of the magnetic flux density to be produced along the axis. In this manner, that portion C of the periodic magnetic field shown in FIG. 1a which is nearest to the uniform field is formed.

In order to eliminate the adverse effects of the counter magnet field upon the periodic field, an annular pole piece 124 is disposed coaxially of the magnet means 121 extending towards the electron-gun-side from the periphery of that pole piece 123 of the uniform magnetic focussing device 12. As has been explained, the arrangement of the invention is such that there are provided the radially outwardly extending pole piece 123 for magnetic shield and annular pole piece 124 attached to the said pole piece for magnetic shield, so as to eliminate the disturbance caused by the counter magnetic field in the region of the junction between the uniform and the periodic magnetic focussing devices and such that the counter magnetic field is positively utilized in production of the periodic magnetic field.

In this embodiment, it is desirable that the axial length of the annular pole piece 124 may be as long as possible in view of its action of magnetically shielding the periodic magnetic focussing device 13 from the uniform magnetic focussing device 12. Too long an annular pole piece 124, however, would result both in decrease in magnetic lines of force Within the space inside of the magnet means 121 and also increase in the magnetic lines of force in the outside space, with the result that the efiiciency of the magnetic means decreases. The axial length of the annular pole piece 124 must be determined in consideration of three points: the above-mentioned shielding effect, the magnetomotive force of the magnet means 121, and the magnetic flux density, required for operating the travelling wave tube, in the inside space of the magnet means 121. The magnetic reluctance of the shunt 15, given by the material, thickness, length, or otherwise, must be determined in view of both the magnetomo-tive force of the cylindrical magnet 131 and the intensity of the counter magnetic field produced by the magnetic means 121. In case the magnetomotive force of the magnet means 121 is small, it is impossible to sufficiently lengthen the axial length of the annular pole piece 124 and to sufliciently magnetically shield the periodic magnetic focussing device 13 from the counter magnetic field of the uniform magnetic focussing device 12. In this case, it is possible to adopt an auxiliary countermeasure to assist cancellation of the effect of the counter magnetic field on the periodic magnetic field :by, as taught in the copending application, alternatingly increasing and decreasing the magnetomotive forces of the cylindrical magnets of the periodic magnetic focussing device or alternatively alternatingly lengthening and shortening the length of the radially inward portions of the pole pieces.

Although only one embodiment and some modifications thereof have so far been described, it will be clearly understood that other modifications are possible within the scope of the invention. Furthermore, the invention is applicable not only to a travelling-wave tube but also to other electron tubes, such as backward-wave tubes, of similar construction. The wording travelling-wave tube in the foregoingdescription and the accompanying claims should therefore be interpreted in its broadest sense. Moreover, the invention is applicable not only to an arrangement where the high-frequency circuit for introducing high-frequency signals to a travelling-Wave tube is a waveguide, but also to a similar arrangement where the high-frequency circuit is a cavity resonator, a coaxial cable, or other types of coupling.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitation to the scope of my invention, as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. A magnetic focussing device for an electron beam consisting of a uniform magnetic focussing device comprised of at least one hollow cylindrical permanent magnet to produce a uniform magnetic field along the axis of said permanent magnet, and a periodic magnetic focussing device comprised of a plurality of aligned hollow cylindrical permanent magnets which are piled so that poles of the same polarity face each other to produce a periodic magnetic field along the axis of said aligned permanent magnets, said uniform and said periodic magnetic focussing devices being joined to each other with their axes in alignment, and a high-frequency circuit interposed between said focussing devices at the site of junction, characterized by a magnetic shielding member provided at said junction for magnetically substantially separating said periodic magnetic focussing device from said uniform magnetic focussing device and a magnetic circuit means including that hollow cylindrical permanent magnet which is nearest to said uniform magnetic focussing device, for producing the periodic magnetic field portion which is nearest to said uniform magnetic focus'sing device in the space enclosed with said high-frequency circuit.

2. An electron :beam magnetic focussing structure for producing a substantially uniform magnetic field over a first portion of its length and a periodically reversing field over a second portion of its length contiguous with said first portion comprising a first magnetic device formed of a hollow magnet, a second magnetic device formed of a plurality of hollow magnets positioned with like poles facing each other, and a spacing between said magnetic devices to accommodate a non-magnetic coupler, having means for producing a first alternation of said second field portion within said space, comprising a sheet of magnetic material intermediate said hollow magnet and said space extending radially outwardly tbeyond said first magnet, and an annular element of magnetic material connected with said sheet and extendingin a direction away from said space.

3. A focussing device according to claim 2, further comprising a magnetic shunt extending from said sheet to the far end of the first magnet of said plurality of magnets.

No references cited.

GEORGE N. WESTBY, Primary Examiner.

R. SEGAL, Assistant Examiner. 

1. A MAGNETIC FOCUSSING DEVICE FOR AN ELECTRON BEAM CONSISTING OF A UNIFORM MAGNETIC FUCUSSING DEVICE COMPRISED OF AT LEAST ONE HOLLOW CYLINDRICAL PERMANENT MAGNET TO PRODUCE A UNIFORM MAGNETIC FIELD ALONG THE AXIS OF SAID PERMANENT MAGNET, AND A PERIODIC MAGNETIC FOCUSSING DEVICE COMPRISED OF A PLURALITY OF ALIGNED HOLLOW CYLINDRICAL PERMANENT MAGNETS WHICH ARE PILED SO THAT POLES OF THE SAME POLARITY FACE EACH OTHER TO PRODUCE A PERIODIC MAGNETIC FIELD ALONG THE AXIS OF SAID ALIGNED PERMANENT MAGNETS, SAID UNIFORM AND SAID PERIODIC MAGNETIC FOCUSSING DEVICES BEING JOINED TO EACH OTHER WITH THEIR AXES IN ALIGNMENT, AND A HIGH-FREQUENCY CIRCUIT INTERPOSED BETWEEN SAID FOCUSSING DEVICES AT THE SITE OF JUNCTION, CHARACTERIZED BY A MAGNETIC SHIELDING MEMBER PROVIDED AT SAID JUNCTION FOR MAGNETICALLY SUBSTANTIALLY SEPARATING SAID PERIODIC MAGNETIC FOCUSSING DEVICE FROM SAID UNIFORM MAGNETIC FOCUSSING DEVICE AND A MAGNETIC CIRCUIT MEANS INCLUDING THAT HOLLOW CYLINDRICAL PERMANENT MAGNET WHICH IS NEAREST TO SAID UNIFORM MAGNETIC FOCUSSING DEVICE, FOR PRODUCING THE PERIODIC MAGNETIC FIELD PORTION WHICH IS NEAREST TO SAID UNIFORM MAGNETIC FOCUSSING DEVICE IN THE SPACE ENCLOSED WITH SAID HIGH-FREQUENCY CIRCUIT. 